We propose to study diversity, somatic reorganization, and expression of mouse Ig genes. Using the recombinant DNA technique, we will clone a variety of DNA fragments containing the Ig genes and their flanking sequences and characterize these DNA clones by various methods including heteroduplex analysis, R loop analysis, restriction enzyme mapping and DNA sequencing. As the sources of cells we will use myelomas, hybridomas representing various stages of B cell differentiation and natural B lymphocytes fractionated by a fluorescence-activated cell sorter. We will also identify and characterize enzymes responsible for somatic recombinations. Our goal is to decipher at the molecular level the genetic mechanisms that are responsible for the vast antibody diversity and the expression of Ig genes including allelic exclusion, heavy chain switch and simultaneous synthesis of two heavy chain classes.